Tuesday, January 19, 2016

THE dramatic events of this past weekend mark a potential turning point in the modern history of the Middle East. Estranged for the last three and a half decades, the American and Iranian governments are talking and working with each other once again.

The implementation of the nuclear deal and the lifting of sanctions on Iran, as well as the prisoner exchange, combined to make it a rare, hopeful day for Washington and Tehran. But Iran remains a powerful adversary of America across nearly all the conflicts of the Middle East. President Obama and his successor in the White House will be tested by whether they can find the right balance between cooperation on nuclear issues and containment of Iranian aggression.

The deal will have multiple consequences. With the lifting of sanctions, Iran becomes a major player in global energy markets. It will also cease to be a political pariah in much of the world. Most important, a possible third major Middle East war has been averted and Iran’s bid for nuclear weapons stymied, at least for now. At a time of upheaval in the region, this is an unusual piece of positive news.

Despite criticism of the agreement from congressional opponents, there are clear benefits for American security. Iran’s nuclear program will be frozen for 10 to 15 years now that its plutonium production and uranium enrichment facilities have been largely dismantled. A vast majority of its enriched uranium has been shipped abroad. Tehran will be subject to tight international supervision and monitoring.

The commitments made by Tehran’s lead negotiator, Foreign Minister Mohammad Javad Zarif, will be difficult to disavow in the future without major costs to Iran’s global reputation. If it tries to trim or cheat on its obligations, the world will be watching.

The return of diplomacy as a preferred tool of American strategy is another of the deal’s most promising results. Mr. Obama and Secretary of State John Kerry can take much credit. Their decision to engage in a risky two-year negotiation with arguably our most difficult regional adversary has paid off. By emphasizing the primacy of diplomacy in the wake of the interventions in Afghanistan and Iraq, Mr. Obama believes he is redefining American leadership in the world in a more constructive and realistic way.

Despite the nuclear deal’s many promises, however, the road to a more normal American-Iranian relationship will be long and rocky. That is how I saw it when I helped to negotiate sanctions against Iran for the Bush administration a decade ago. It remains true today.

One immediate challenge will be to deal with two Iranian governments at once. Mr. Kerry pounded out the agreement with the American-educated Mr. Zarif, who was backed by Iran’s reformist president, Hassan Rouhani. But real power still rests with Iran’s supreme leader, Ayatollah Ali Khamenei, a recluse who is supremely distrustful of all things American and closer to the hard-line Islamic Revolutionary Guards Corps than to the reformists.

The guards corps’s influence over Iran’s national security strategy has been visible since the nuclear deal was announced. In recent weeks, Iran tested ballistic missiles in defiance of United Nations Security Council resolutions and fired rockets close to American warships in the Strait of Hormuz. After detaining American sailors last week, Iran released a demeaning video of the incident.

The guards’ corps is also driving Iran’s continued support for Houthi rebels in Yemen, Hezbollah in Lebanon and Syria and the bloody government of President Bashar al-Assad. It is the guards’ corps that may be tempted to cheat on Iran’s nuclear obligations and return to taking American hostages.

Dealing with these rival camps will be a challenge. Mr. Kerry is right to test whether Iran will be more cooperative in the negotiations to end the war in Syria and in the fight against the Islamic State. But the administration will need to be equally determined to contain the worst aspects of Iran’s aggression in the region.

The president made a start Sunday by announcing sanctions against Iran for its missile tests. Mr. Obama will also need to repair our damaged relationships with Israel and Saudi Arabia to limit the mayhem Iran may yet cause in Syria. He should also counter any Iranian attempt to encourage its clients Hezbollah and Hamas to resume rocket attacks on Israel.

This will be a difficult set of issues to manage in a polarized election year. Is it too much to hope that some G.O.P. leaders might drop their opposition and help implement the deal? Or that more Democrats will back a tough-minded approach to Iran’s troublemaking?

One thing is clear: The United States will face a major strategic challenge from Iran this year and next. In order to realize the long-term benefits of this historic deal, the United States must continue to balance engagement with deterrence.

Nicholas Burns, a professor at the Harvard Kennedy School and a former undersecretary of state, is a visiting fellow at Stanford.